This invention relates generally to a voltage monitor for a magnetically insulated transmission line (MITL), and more particularly to a probe for monitoring MITL voltage as a function of probe current.
Voltage is a very important parameter for the operation of a Terawatt class pulser. For example, a voltage measurement is crucial to the study of pulser power flow and time-dependent load behavior. In addition, a voltage measurement near a MITL load reduces the magnitude of the inductive voltage correction needed to determine the voltage at the load. Furthermore, if voltage is measured in the water section of the pulser, voltage corrections from the water section to the load are affected by the vacuum insulator stack region, which region often supports non-TEM waves, and by insulators that may partially flash and the ultimately short completely during a power pulse. The erratic behavior of a highly stressed insulator is a severe hindrance to accurately extrapolating voltage measured in the water to the voltage at the load.
This load voltage has proven extremely difficult to measure accurately. Ideally, a purely electrical diagnostic can provide good resolution with a single data channel. Standard electrical techniques are the inductive voltage monitor, resistive dividers, electro-optical devices, and capacitive monitors. Each of these has some significant disadvantages.
The inductive voltage monitor shunts the transmission line with an inductor of known value, and the rate of change of magnetic flux is measured. This technique can be used in vacuum, and provides large signal levels. The problem is the power flow disruption that results from this current shunt. First, the current in the inductor rises with the integral of the voltage across it, which means that current loss increases as the power pulse progresses. The measuring inductor's resulting falling ratio of voltage-to-current can excessively load the pulser late in the power pulse. A higher inductance monitor lessens this loss, but it requires a physically larger inductor which, in turn, inhibits high-frequency response because of transit-time effects. The second major problem with the inductive divider in a MITL is the electron loss resulting from the magnetic null if the monitor bifurcates the power flow, as it usually does. The magnetic null is created because the monitor must be self-magnetically insulated, and downstream of the monitor this magnetic field is oppositely directed from the main MITL field. Therefore, there must be a magnetic null at some point downstream of the monitor. Electron losses at this null can be substantial.
Resistive dividers have been used in vacuum, but flashover of the resistor surface due to the high voltage and electron bombardment is an overwhelming problem at voltages exceeding a few MV in low impedance systems.
Electro-optical electric field measurement techniques offer the advantage of immunity to electrical noise. This technique has been used to measure voltage in the water section of a TW pulser, but the problem of radiation scintillation in fiber-optic cables (from X-rays, electrons, ions, and neutrons) has hindered the application of this technique to the vacuum section of TW pulsers.
Capacitive voltage monitors are commonly used in the oil and water sections of large pulsers. Capacitive probes function well in oil and water, as long as there is no breakdown in the dielectric liquid which would distort the electric field. There are two serious practical concerns in using capacitive monitors in a MITL. The first is that the cathode electric field in a MITL is essentially zero. Therefore, the probe must be placed in the anode where the probe and associated signal cabling are subject to energetic electron bombardment, a problem because complete shielding for signal cables is difficult for MeV electrons. The second concern is the small signal level of the displacement current signal, and the fact that the displacement current collector itself will also collect electron charge. A MITL has an electron loss front during the beginning of the power pulse, and upstream perturbations can launch unstable flows that reach the anode throughout the power pulse. Though these losses are typically small compared to the current delivered to the load, the electron loss current can dominate the displacement current collected by the capacitive monitor.
A high-B field vacuum capacitive voltage monitor has also been developed. The construction of this monitor is quite complicated, and it is quite sensitive to poor vacuum because its magnetic field configuration can retain ionizing electrons.
Voltage can also be determined knowing anode and cathode currents, and the vacuum impedance of the MITL. This has been done on systems where the vacuum electron flow is a substantial fraction of total current, such as 20% or greater. However, since efficient systems have a minimum vacuum electron flow, usually less than 3%, the measurement is difficult because the voltage is computed from the difference in anode and cathode currents.